This invention relates to a ground fault current protective system, or ground fault circuit interrupter, for use in an electrical distribution circuit with a grounded neutral conductor, and more particularly relates to such a ground fault protective system which detects and responds to low impedance ground faults on the neutral conductor, which occur on the load side of the ground fault protective system, by partial or total attenuation of an output signal of an oscillator which is responsive to the impedance level in the neutral conductor.
After the introduction of ground fault protective systems using differential transformers which were intended to protect people against ground fault currents as low as five milliamperes, it was discovered that such systems had difficulty responding to line conductor to ground faults when a low impedance ground fault existed on the neutral conductor between the load and the ground fault detector. This problem occurs because a low impedance ground fault on the neutral conductor results in the neutral conductor becoming a shorted or low impedance winding of the differential transformer. Such a low impedance winding, under ground fault conditions on the line conductor, diverts most of the magnetic energy of the differential transformer from its secondary winding leaving little or no magnetic energy for providing a signal to interrupt the electrical distribution circuit.
One previously proposed solution to the low impedance neutral conductor problem is to insert an impedance in the neutral conductor between the ground fault sensing means and the load as is disclosed in U.S. Pat. No. 3,473,091 issued on Oct. 14, 1969 to A. R. Morris et al. Another prior solution is disclosed in U.S. Pat. No. 3,611,035 issued on Oct. 5, 1971 to E. S. Douglas. Here a high frequency tickler voltage is induced onto the neutral conductor to cause a current imbalance in the line and neutral conductors which is detected by the ground fault sensing means when a ground fault occurs on the neutral conductor.